Cystine-Binding Thiol Drugs
Cystine-binding thiol drugs (CBTDs) are another approach in treating cystinuria. Two main CBTDs, tiopronin and D-penicillamine, bind to cystine in the urine, reduce the disulfide bond and ultimately produce drug-bound cysteine complexes, which are up to 50 times more soluble than cystine. There are tolerability issues with both drugs so the patients developing proteinuria, anaemia, leucopenia, and liver dysfunction need to be monitored. Cystine capacity (CysCap) is a novel assay developed by Litholink (Chicago, Illinois, United States of America) for the prediction of kidney stone events in cystinuria. The solid-phase assay measures amounts of cystine crystals after incubation of urine samples for 48 hours with a pre-formed amount of cystine crystals. In supersaturated urine, the recovered solid phase is greater than the one that was added. Such urine has a ‘negative cystine capacity (CysCap)’. A ’Positive CysCap’ urine can dissolve the added cystine crystals. A patient with a positive CysCap has less stone events.
The CysCap test is used to judge the effectiveness of CBTD tiopronin. The tiopronin dose-response study showed that the CysCap increased in tiopronin treated patients when the dose of 1g per day was applied. However, there was no consistent further increase in CysCap or in cystine excretion within 24 h with higher tiopronin doses .
In addition to CysCap test, crystalluria and crystal volume are identified by microscopic examination of urine sediment. The retrospective study, performed in order to identify therapeutics targets using cystine crystalluria as a marker of cystine stone development, showed that a decrease in urine specific gravity substantially decreased the risk of cystine crystalluria .
Chaperone Therapy, Crystal Inhibitors and Other Approaches
Chaperone therapy is an attractive treatment for cystinuria since it is already applied on several mutations affecting the rBAT protein that cause protein misfolding.
Specially designed synthetic inhibitors of crystal growth are known to modulate crystallisation in urine, and could be able to slow or prevent cystine stones. As the stones are aggregates of individual hexagonal crystals and they generate hillocks in a spiral growth pattern, it is shown that L-cystine dimethyl ester (L-CDME) and L-cystine methyl ester (L-CME) significantly decrease the growth velocity of hillocks steps. L-CDME inhibitor affects the roughening of the crystal step edges in a higher rate than L-CME. L-CDME is indicated to be a viable therapeutic agent for the prevention of L-cystine stones . In order to assess the efficacy of L-CDME as an inhibitor of crystal growth, the Slc3a1 knockout mice were given either water or L-CDME for four weeks. The treatment with L-CDME led to formation of high number of small stones but it did not prevent stone formation. Ultimately, the study showed that L-CDME could be used as an applicable treatment for cystine stones without any negative effects on the mice .
Other approaches include the vasopressin V2 receptor antagonist (V2RA) tolvaptan and the application of CRISPR/Cas9 gene editing systems. Tolvaptan causes polyuria, with urine output that averages six litres per day and as such is an attractive pharmacotherapeutic for cystine stones . In an effort to apply gene therapy to repair SLC7A9 mutation by delivery of CRISPR/Cas9 reagents, there are numerous barriers to be overcome.